A common type of spring-operated brake actuator used on heavy trucks and other commercial vehicles utilizes a powerful compression spring to provide the required braking force when the air pressure inside a pressurized chamber falls below a predetermined minimum, for example when the vehicle is parked or there is a malfunction in the air-operated service brake system. The spring is located between an end wall of a brake head housing and the pressurized chamber, with a flexible diaphragm extending across the interior of the housing and dividing it into an unpressurized spring chamber and a pressurized air chamber. The force exerted on the diaphragm by the pressurized air exerts a counter-force on the spring, which holds it in a compressed position inside the spring chamber. When the pressure inside the air chamber drops, the unbalanced force exerted by the compression spring is transmitted through the diaphragm to an actuator rod that extends from the pressurized side of the diaphragm to the exterior of the actuator. In a so-called "double diaphragm" spring brake actuator, a service brake housing is provided in line with the spring brake housing, and a second diaphragm inside the service brake housing transits a modulated force to the actuator rod during normal operation of the service brake system.
Because the spring brake assembly is a critical safety system, it has to be designed and manufactured for exceptional reliability. Moreover, because the compression spring inside the spring actuator must exert a sufficient force to apply a maximum braking force to the vehicle independent of any external air pressure assistance from the service brake system, the spring in its compressed state stores sufficient kinetic energy to be lethal if the spring brake housing is opened without first taking adequate precautions to secure the spring in its compressed state or to release the stored energy in a controlled manner.
It is conventional to fabricate the spring brake housing containing the compression spring and diaphragm in two opposing sections (an aluminum adapter housing and a steel cap) separated by the periphery of the diaphragm, with the two opposing sections being clamped together with sufficient force to maintain an air-tight seal between the diaphragm and the housing. Since unauthorized opening of the housing is potentially dangerous to the inexperienced mechanic, and since unauthorized re-sealing using defective seals or improper procedures could result in a poor diaphragm seal or other internal defect, it is also conventional to discourage unauthorized access, disassembly, and reassembly of the housing by appending a warning label to the housing and using a clamping means which is tamper-resistant or at least tamper- o evident, so that a new clamping means, not readily available to unauthorized personnel, is required before the housing can be reassembled.
Despite these relatively obvious safety precautions, the known clamping means can be removed by simple handtools and the warning labels are exposed to dirt and road hazards and are thus effective only when the assembly is relatively new and not likely to require inspection or repair. Thus, there is a need for an improved spring brake housing that is even less prone to unauthorized disassembly and its consequent hazards.
Another disadvantage of the clamping means used in the prior art is that they present cavities and crevices in which moisture and debris can accumulate, which may eventually cause corrosion of the housing in the vicinity of the seal between the housing and the diaphragm, which in turn may lead to premature failure of the seal, or even of the diaphragm itself, requiring early replacement or rebuilding of the entire spring brake assembly.
As an alternative to a separate conventional clamping means, it has also been proposed to form an annular metal retaining member as an integral part of the steel cap, which is then deformed beyond its elastic limit around an annular flange on the aluminum adapter housing. However, such an alternative construction does not permit a major structural component of the spring brake assembly (the steel cap) to be reused, and is thus relatively expensive and environmentally undesirable.